It has 5V stepdown converter and Li-Po charger as well as amplifier integrated on board. Cheap USB-C Li-Po chargers lack neccessary 5.1kOhm pull-downs and therefore work only with USB-A->USB-C cables. Soldering them onto charger PCB is borderline impossible, while adding them to most USB-C sockets is fairly easy (usually there are free pads to solder 5.1k SMD resistor). You then connect 5V and GND from USB-C socket to BT module and it handles charging the battery that is connected to separate pads.
I don't really understand why article recommends board without amplifier, I tried using CSR8675 board without one and it was waaay too quiet.
With 500mAh battery salvaged from old BT speaker I get over 20h of loud playback, which is fine for me.
[1] https://www.reddit.com/r/headphones/comments/r32ru2/yet_anot...